Integrated circuit for storage and retrieval of multiple digital bits per nonvolatile memory cell

ABSTRACT

An integrated circuit storing multiple bits per memory cell is described. The amount of charge stored in a memory cell corresponds the multiple bits in a memory cell. Dual banks of shift registers are alternately coupled to one or more data pins and to the memory cells of the memory array speed data transfer for reading and writing operation. Reading is performed in the voltage mode to conserve power. During writing operations, reading of a memory cell is performed in the voltage mode to determine whether the desired programming of the memory cell has been achieved. During the reading of a memory cell, the voltage corresponding to the amount of charge stored in a memory cell is compared against a binary search sequence of reference voltages to determine the multiple bits stored in the memory cell.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of Ser. No. 08/867,350, filed onJun. 2, 1997, now U.S. Pat. No. 5,905,673, which is a continuation ofSer. No. 08/540,117, filed on Oct. 6, 1995, now U.S. Pat. No. 5,687,114.

FIELD OF THE INVENTION

This invention-relates in general to semiconductor memories and, inparticular, to nonvolatile semiconductor memories with the ability tostore multiple digital bits per memory cell.

BACKGROUND OF THE INVENTION

Nonvolatile semiconductor memories, such as EEPROM, EPROM and FLASHintegrated circuits, have traditionally been used to store a singledigital bit per memory cell. This has been done by changing thethreshold voltage (conduction) characteristics of the cell by retaininga certain amount of charge on the floating gate of the memory cell. Thethreshold voltage range is normally partitioned into two levels(conducting versus nonconducting) to represent the storage of onedigital bit per memory cell.

A wide range of charge can be reliably stored on the floating gate torepresent a range of threshold voltages. Charge retention on thefloating gate can be partitioned to represent multiple number ofthreshold voltage ranges and the threshold range can be partitioned intomultiple ranges to represent storage of more than one bit of digitaldata per memory cell. For example, four threshold partitions can be usedto represent storage of two digital bits per memory location and sixteenpartitions to represent storage of four digital bits per memorylocation. Furthermore, the threshold voltage range can be partitioned toappropriately finer resolution to represent the direct storage of analoginformation per memory cell.

The ability to store multiple digital bits per memory cell increases theeffective storage density per unit area and reduces the cost of storageper digital bit. In addition to this, in the field of semiconductormemories, the costs of a-modern fabrication facility often exceeds abillion dollars. Application of multibit storage per cell techniques toexisting memory fabrication processes and facilities allows theproduction of the next generation of higher density storage devices inthe same manufacturing facilities, thereby increasing profitability andthe return on investment.

Nonetheless, the problem of operational speed, i.e., the reading andwriting operations, have yet to be satisfactorily addressed for deviceshaving multiple bits per memory cell. A related problem is powerdissipation. As more power is used to increase operational speeds, powerconsumption is also undesirably increased. Still another problem isreliability. While charges can be stored in the floating gates of memorycells for very long periods, erasing and rewriting charges causes longterm problems as to the certainty of the bits stored in a memory cell.And, of course, any integrated circuit has problems of space. In anintegrated circuit having multiple bits per cell, additional circuitsmust be added to handle the new requirements. This partially negates theadvantages of the increased bits per memory cell.

The present invention solves or substantially mitigates these problems.The present invention speeds up the reading and writing operations ofmultibit memory cells. Power dissipation is lowered for readingoperations. The present invention also permits the reliabledetermination of the bits in the memory cells over the long term andalso conserves space on the integrated circuit.

SUMMARY OF THE INVENTION

The present invention provides for an integrated circuit having an arrayof memory cells, each memory cell storing multiple bits of information,end at least one data terminal. The integrated circuit also has aplurality of latches connected to the array of memory cells with thelatches organized into a first bank and a second bank. For reading andwriting operations from and into the memory cell array, the latches andmemory cell array are controlled so the first bank is coupled to thearray of memory cells while the second bank is coupled to the dataterminal. Alternately the second bank to the array of memory cells whilefirst bank is coupled to said one data terminal. This alternate couplingpermits data to be simultaneously transferred between one bank oflatches and the array of memory cells and transferred between anotherbank of latches and the data terminal for faster read and writeoperations.

To lower power dissipation, the memory cells of the array are read byvoltage-mode operation. Furthermore, during writing operations, avoltage corresponding to the amount of charge stored in the selectedmemory cell is compared to a reference voltage to determine whether highvoltage programming of the memory cell should continue. Programming ofthe memory cells is terminated when the corresponding voltage matchesthe reference voltages.

For reading operations, the voltage corresponding to the amount ofcharge stored in a selected memory cell is compared to a sequence ofreference voltages in a binary search pattern to determine the pluralityof bits stored in the memory cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the major circuit blocksimplemented on a single integrated circuit chip according to the presentinvention;

FIG. 2A shows a circuit generally illustrating current-mode reading ofthe memory cells in FIG. 1; likewise,

FIG. 2B shows a circuit generally illustrating voltage-mode reading ofthe memory cells in FIG. 1;

FIG. 3 shows the organization of the reference cells and array cellswithin a block and the connection of the threshold partition voltagereference generation blocks to their respective arrays;

FIG. 4 is a block diagram of the multilevel dual mode shift registers inFIG. 1;

FIG. 5 illustrates the general organization of two Y-drivers in FIG. 1;

FIG. 6 shows details of the multilevel dual shift registers in FIG. 4and circuitry that allow the dual shift registers to be used both duringwriting and reading operations;

FIG. 7 illustrates the reference multiplexer circuit in FIG. 5 for eachY-driver;

FIG. 8A shows the circuit details of the voltage comparator, the latch,the program and read control block and the high voltage switch, whichare common to each Y-driver; FIG. 8B shows the circuit level detail ofthe voltage comparator, the latch, the program and read control blockand the high voltage switch and the read mode path for the referenceY-drivers with additional circuitry which allow all reference cells in ablock to be read in parallel; and FIG. 8C shows the details of theY-multiplexer circuit of a reference Y-driver and Y-multiplexers;

FIG. 9A shows details of the Y-multiplexer common to all Y-drivers, theX-Decoder block, the X-multiplexer common to each X-decoder and memorycells common to one Y-driver and one X-decoder with connections to thereference Y-multiplexer and reference cell array; and FIG. 9B shows thecircuit of a single transistor memory cell according to one embodimentof the present invention;

FIG. 10 is a scale from 0 Volts to Vmax Volts of the various programthreshold partition voltages for the reference memory cells and the datastorage memory cells;

FIG. 11 details the threshold partition voltage reference generationblocks; and

FIG. 12A represents the tree decoding in the binary search algorithm ina read operation to determine the digital bits stored a selected memorycell; and FIG. 12B is a flow chart for the binary search algorithm for aread operation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It should be noted that the drawings have the elements with samereference numeral. This emphasizes the similar structure or operation ofthe elements. Furthermore, the symbol for a MOS transistor has beensomewhat modified to a straight line representing the source and drainof the transistor and a short line parallel to the source/drain line torepresent the gate of the transistor.

General Description of the Integrated Circuit

The major blocks of a preferred embodiment of the present invention areshown in FIG. 1. A nonvolatile memory array 1 and a reference memoryarray 2 has memory cells connected in a two dimensional array of rowsand columns. The memory cells can be any of the existing devicearchitectures, such as, for example, EPROM, EEPROM, FLASH, or existingcell structures, such as single transistor, two transistor, split-gate,NAND, AND, and DINOR cell structures, or ground array architecture,including standard and virtual ground, known in prior art. Depending onwhich device architecture, cell structure or ground array architecture,is chosen, specific programming, erase and read algorithms can be easilydeveloped, including the specific voltages required at each of theelectrical terminals of the cell to facilitate the storage of more thanone digital bit per nonvolatile memory cell. A cell can hold more thanone nonvolatile device, for example, a NAND, DINOR or AND cellstructure, already known in prior art. The specifics of the device,array architecture or cell structure and algorithms are not part of thepresent invention.

Each of the memory arrays 1 and 2 are further organized into blocks,having single or multiple rows. Each block consists of all of thecolumns or part of the columns of the arrays 1 and 2. In FIG. 1 a memoryblock is shown with all the columns in a single row. Each memory blockconsists of the cells from the reference array 2 and the cells from thememory array 1.

An error correction array 3 has nonvolatile memory cells similar tothose used in the memory 1 and the reference array 2. In one embodiment,the error correction array 3 contains additional coding informationrequired for an on-chip Error Correcting Code (ECC) mechanism, as isknown in prior art for ECC implementation. In another embodiment, theerror correction array 3 contains the full address of defective cellswhich should be avoided during a write or a read operation. The size ofthe error correction array 3 depends on the maximum number of defectivecells which may be corrected. During a production verification phase,the memory array 1 is tested to identify any defective cells. Theaddress of these defective cells are programmed into the errorcorrection array 3 before the chip is shipped from the factory. Theerror correction array 3 may be programmed using more than one bit permemory cell or may be programmed using a single bit per cell. If ECCcorrection is implemented, the error correction array 3 is automaticallyloaded with coding bits with on-chip ECC circuitry. An error correctioncontrol and logic block 16 contains all the necessary addressing,decoding and sequencing circuitry necessary to implement either one ofthe error corrections embodiments mentioned above.

A memory management array 4 contains address information for the blocksthat are available for further writing at a certain time and alsophysical address information for blocks during sequential writing orreading of multiple blocks which are not necessarily physicallycontiguous in the memory array but are logically contiguous. Memorymanagement of the array improves the long term reliability of theproduct and also allows for more efficient use of the memory inenvironments where serial data of variable length are frequently erasedand rewritten. In such operations, only the beginning and ending blockaddresses are provided and the data is accessed through clocking.Instead of providing the ending block address, a stop signal can also beused to signify the end of the variable block serial data. The mode iscalled the “serial write and read access” mode and is generally used fordigital audio record and playback systems, and also for semiconductormemory systems which replace mechanical disks. The serial write and readaccess mode with error correction and memory management allows thepresent invention to substitute integrated circuit memory for digitalaudio record and playback systems and also for general digital datastorage systems. A memory management logic block 24 contains thenecessary sequencing circuitry to perform the memory management functionin conjunction with the memory management array 4. The data in thememory management array 4 may be simply a single bit per memory cell ormore than one digital bit per cell as in the arrays 1 and 2.

A redundancy block 5 has additional blocks of memory cells that can beused to repair whole blocks of cells which cannot be used. This sort ofblock redundancy is known to designers of memory integrated circuits.The number of blocks in the redundancy block 5 defines the maximumnumber of blocks that can be repaired either during the productionverification phase or in the field during an embedded repair phase.

Addressing of the memory cells of the array 1 is provided by an addressdecoder 13 which is coupled to a serial interface block 14 which isconnected to the external world. The decoded addresses are passed to aY-counter block 12 and an X-counter block 11 from the decoder 13. Theoutput from the Y-counter block 12 is passed to a Y-multiplexer block 8which selects the desired block of memory cells in the array 1. Theoutput of the X-counter block 11 is decoded in the X-decoder block 7 andthe X-multiplexer block 6, to select the desired row in the selectedblock in the memory array 1.

The address decode block 13 generates the starting address of a selectedrow. The decoded address is set into the X-counter 11 and Y-counter 12at the beginning of each new access operation of a certain length ofdata stream. After the starting address is provided, data is seriallyaccessed by a clock input to the chip. The serial interface block 14contains the circuitry required to perform the appropriate serialprotocol with other external chips. The serial protocol can be any ofthe industry standard serial protocols or a proprietary protocol.Generic serial interface signals are shown in FIG. 1 going in and out ofthe serial interface block 14.

The X-counter block 11 contains digital counters which increment theircount by a clock signal YOUT, the output of the Y-counter block 12, on aline 27. The Y-counter block 12 is clocked by a signal CLCK on an inputline 28 and generates a clock signal SHFT CLK on a line 29 to thevarious sections of the Y-drivers. The Y-counter block 12, in turn,provides the clock signal YOUT on the line 27 to the X-counter block 11.

The X-multiplexer block 6 provides the output of one X-decoder stage inan X-decoder block 7 on a selective basis to multiple rows of the array.This accommodates the circuitry of an X-decoder without letting theaspect ratio of the integrated circuit layout of the X-decoder frombecoming inordinately large. X-multiplexers and their use are known inthe prior art. The X-decoder block 7 contains the X-decoders which areused to select the rows of the. memory array 1 and 2. Details about theX-decoder block 7 and the X-multiplexer block 6 are provided below andare also shown in FIG. 9A. The Y-multiplexer block 8, similar to theX-multiplexer block 6, selects the output of one of the Y-drivers,described in more detail below, and provides it on a selective basis toany one of a number of columns of the memory array. This is again doneto fit the pitch of the Y-drivers and the memory array in the columndirection.

A read-write circuit block 9 contains the necessary circuitry to performhigh-voltage write and low-voltage read operations of data to and fromthe array 1. Details about the read-write block 9 are provided below.

A multilevel dual-shift register block 10 which has serially connectedlatches lies between the data input and output terminals and the memoryarray 1 and 2. Data to be written into the memory array 1 is movedserially through a DATA IN 25 line to the block 10 to the memory array1. Data to be read from the memory array 1 is moved from the memoryarray 1 to the block 10 and then transferred serially from the block 10through a DATA OUT 26 line. A detailed description is provided below. Asystem control logic block 15 contains the necessary control andsequencing circuitry to allow proper system operation. A test modecontrol and logic 17 block contains circuitry that allow full functionaltesting of the chip. Through the use of test modes, the chip isreconfigured into various alternative test configurations that allowfaster and more efficient verification of the chip. These test modes arenormally accessed in the verification phase at the factory but certaintest modes may also be accessed in the field, such as for array repairtest modes using the redundancy block 5.

A program/erase/read algorithm block 18 provides all the control andsequencing signals to perform the intelligent programming, erasing andreading of digital data from the memory array 1.

An oscillator block 19 generates clock signals for the high voltagegeneration and also provides clock signals for the program/erase/readalgorithm block 18 and also for other system clocking andsynchronization purposes. Alternatively, if the oscillator block 19 isnot placed on-chip, then its output signals must be supplied externallyto the integrated circuit.

A charge pump 20 block generates high voltages on-chip. A high voltageshaping and control block 21 receives the output signal of the chargepump block 20 and properly shapes the high voltage pulses withpredetermined rise and fall times. High voltage pulse shaping iscritical for long term reliability of the operation of the integratedcircuit. High voltages shaped pulses can also be provided externally. Orunshaped high voltage can be provided from an external source, which canbe then be pulse shaped with the proper rise and fall times with on-chipcircuitry.

A nonvolatile scratch pad memory and registers block 22 has memory cellssimilar to those in the nonvolatile memory array 1. These memory cellsare suitably organized and are normally used for external system housekeeping and feature requirements. In an audio record and playbacksystem, for example, the nonvolatile scratch pad memory and registersblock 22 contains the information for the number of messages and thetime at which these messages were recorded. Data in the scratch padmemory and registers can be stored as single or multibit per memorycell.

An on-chip bandgap reference block 23 generates the necessary analogvoltage and current references required for operation of the integratedcircuit. These voltage and current references are used to providereference voltages and currents which are compensated for temperatureand power supply variations. System performance is stabilized over widetemperature and power supply ranges.

General Reading Operation of a Memory Cell

Heretofore, current-mode operation is typically discussed for thereading of multiple bits per memory cell. Current-mode reading has anadvantage of fast access times. FIG. 2A illustrates the general circuitarrangements for reading in the current-mode, using a single transistormemory cell. This general topology is applicable to othercell-structures too.

A nonvolatile memory cell 30 is typically connected in an inverter mode.The voltage Vs at the source 31 of the transistor, which forms the cell30, is connected to ground. The control gate 36 of the memory cell 30 isconnected to a suitable voltage, Vg, or switched to the power sourcevoltage. The drain of the memory cell 30, which also forms part of acolumn line 32 of the memory array of which the memory cell 30 is apart, normally is connected to a current sense amplifier 33. Thenonvolatile memory cell 30 is connected to the column line 32 throughsome selection circuitry (not shown here for simplicity's sake). Thecurrent sense amplifier 33 typically is also connected to a referencecurrent input line 34 for comparison purposes. The result of thecomparison between the column line 32 current through the nonvolatilememory cell 30 and the reference current line 34 is generated as a logiclevel at the logic output line 35.

For a single bit per cell, the simple absence or presence of currentthrough the memory cell 30 is determined. For multibits per memory cell,the amount of current passing through the cell 35 is compared against aset number of currents by changing the reference current at the inputline 34. The signal at the logic output 35 is then decoded to determinethe stored bits. For example, U.S. Pat. No. 5,172,338 by Mehrotra et al.teaches multibit reading schemes using current-mode reading and alsoshows various alternative embodiments. However, while current-modereading could be used in the present invention, reading of memory cellsin the voltage-mode is preferred. This lowers power consumption comparedto the current-mode technique and renders the multibit per cell memoryintegrated circuit more appropriate for low power, relatively sloweraccess applications, such as audio record and playback systems, andmechanical magnetic disk replacement systems.

In voltage-mode reading, the nonvolatile memory cell 30 is connected inthe source follower mode, as illustrated in FIG. 2B, using a singletransistor memory cell. The general voltage-mode topology is alsoapplicable to other cell structures. The source 31 of the transistorforming the cell 30 is connected to a regulated supply at voltage Vsfrom a stable voltage reference, such as a bandgap reference. Thecontrol gate 36 is also connected to the same supply voltage as thesource 31 or a voltage that is high enough to allow the accurate readingof the highest expected voltage Vd at the drain of the cell 30. A stablefixed bias current circuit 37 is connected between ground and thetransistor's drain, which also forms part of the column line 32 of thememory array, as in FIG. 2A. The amount of fixed bias current is small,in the range of 0.5 microampere to 5.0 microampre. This small currentprevents undue cumulative trapping of electrons during multiple readcycles, thereby preventing false readings of the memory cell 30. Thevoltage at the drain, which is also connected to the column line 32through selection circuitry (not shown here), is equal to Vg−Vgd, whereVgd is the gate-to-drain voltage of the memory cell 30 required tosource the current drawn by the bias current circuit 37. The drain ofthe transistor, part of the column line 32, is connected to an inputterminal of a voltage sense amplifier 38. The voltage sense amplifier 38also has a reference voltage input line 39 and a logic out put line 40.Voltages at the transistor drain, the column line 32, and the referencevoltage line 39 are compared and the resulting logic output signals areprovided at the logic output line 40. The current required for readingin the voltage-mode is much less than for the current-mode. Thus readingin the voltage-mode has lower power dissipation.

The voltage read out at the line 32 depends on the amount of negativecharge (electrons) on the floating gate 36 of the non-volatile memorycell 30. A large amount of charge on the floating gate increases thethreshold voltage of the cell 30. The higher threshold voltage increasesthe gate-to-drain voltage Vgd of the cell 30. The voltage at the line 32is then lower with respect to ground. Conversely, when the amount ofcharge in the floating gate is low; the threshold voltage of the cell 30is lowered and the Vgd is decreased. The voltage at the line 32 is thenhigher with respect to ground. By controlling the amount of charge onthe floating gate, suitable read back voltages are generated at the line32. The process of injecting negative charge (electrons) into thefloating gate is referred to as “erasing” and the process of removingcharge from floating gate is referred to as “programming” the floatinggate or memory cell.

During a multibit reading from a single memory cell, the voltage at thetransistor's drain is compared to various voltages at the referencevoltage line 39. The logic output at the line 40 is then decoded toprovide the appropriate bits. With the source follower connection of thememory cell 30, data access is slowed because the whole column line 32must be pulled up through the small memory cell. For certainapplications, this slower access rate is acceptable. As described below,the multilevel dual shift registers effectively improve the read accesstimes.

Organization of the Memory Arrays

FIG. 3 illustrates the organization of the nonvolatile memory array 1and the nonvolatile reference array 2. The memory cells in the referencememory array 2 are used to generate the comparison reference voltagesfor a voltage sense amplifier to determine the bits stored in the memorycells selected in the array 1. In the preferred embodiment describedhere, four bits are stored per memory cell of each array 1 and 2. Asmentioned previously, each block in the preferred embodiment consists ofa row. Each row consists of reference memory cells and array memorycells. All the cells in a row are erased simultaneously, and dependingon the Y-multiplexer multiplexing scheme only part of the row isprogrammed and read from simultaneously. Since four bits are stored permemory cell, there are sixteen reference memory cells per row. In thisembodiment, each Y-driver drives eight memory cells so there are twoY-drivers 42 for a row of sixteen cells in the reference array 2. TheseY-drivers 42 are labeled REFY-DRIVERS. In FIG. 3 only three Y-drivers 41for the memory array 1 are illustrated. There are M Y-drivers 41. Thethree memory array Y-drivers shown are labeled Y-DRIVER0 to Y-DRIVER2. Areference threshold partition voltage generation block 44, part of thebandgap reference block 23 of FIG. 1, drives sixteen reference lines,each with one of the reference voltages REFB0-REFB15, into theREFY-DRIVERS 42 and an array threshold-partition voltage generationblock 43, also part of the block 23 of FIG. 1, drives the sixteenreference lines, each with one of the reference voltages REFA0-REFA15,into the array Y-drivers 41. The voltage relationships between theREFA0-15 and REFB0-15 signals is shown in FIG. 10.

During a writing operation, a WRITE signal on WR line 46 is high, whichturns on a set of N-channel transistors 45 (outlined by a dashedrectangle). The sixteen REFA015 reference voltages of the block 43 arepassed to-the Y-driver reference voltage lines, RFL015. These referencelevel voltages, REFA0-REFA15, from the block 43 are selectivelyprogrammed into the memory array 1 cells. Likewise, the referencevoltages, REFB0-REFB15, from the block 44 are selectively programmedinto the reference cells of the array 2.

During a reading operation, the WRITE signal on the WR line 46 is drivenlow to turn off the transistors 45. Instead, a set of transistors 47(also outlined by a dashed rectangle) are turned on to pass thereference REFB0-15 output voltages stored in the reference cells of thearray 2 to the Y-driver 41 reference voltage lines, RFL015. TheREFB0-REFB15 voltages stored and read back from the cells of thereference array 2 are used as reference voltages to ascertain thedigital bits stored in the cells of the memory array 1 through a binarysearch technique described below. The use of reference cells per block,or row as in the preferred embodiment, cancels power supply andtemperature variations by placing such variations in the common mode.The memory cells in both array 1 and 2 are subject to the samevariations. The reference cells in array 2 are also subject to the samenumber of program and erase cycles as that of the memory cells in thearray 1, thereby placing the long term aging effects of the cells in ablock or row in the common mode. This reference mechanism has theadvantage of lower current read back mode and allows for longer andbetter long term reliability and accurate read back of digital bits,compared to previously described techniques. The on-chip thresholdvoltage generation (temperature and power supply compensated) blocks 44and 43 also create higher reliability compared to prior efforts in thisfield. The blocks 44 and 43 do not use nonvolatile memory cells togenerate threshold partition voltages, but rather depend on much morereliable and stable components, such as resistors, operationalamplifiers and bandgap voltage sources. Thus the present invention hasimproved long term reliability and accuracy, and stability overtemperature and power supply variations.

In another embodiment of the present invention, the cells of thereference array 2 are first programmed. Then the output of theprogrammed reference cells from the array 2 are used to selectivelyprogram the cells of the memory array 1, with an offset to place theprogrammed levels midway between the programmed reference levels, asindicated in FIG. 10. This method does not require the block 43 butrequires additional time to program the reference cells first.

Dual Shift Registers for Data

FIG. 4 is a block level representation of the multilevel dual shiftregisters block 10, shown in FIG. 1 and part of each of the Y-drivers 41of FIG. 3. The multilevel dual shift register block 10 has latches whichare organized into two banks, A and B. Each bank of latches is connectedserially to form a large shift register. Each bank has four latches foreach Y-driver 41. In FIG. 5, for each Y-driver 41, during a writingoperation the data enters serially through the dual shift registers ofblock 10 and during reading operations the data exits serially throughthe dual shift registers of block 10. The data information travels fromtop to bottom within each Y-driver 41 during writing operations and frombottom to top during reading operations. In general, signals common toall Y-drivers 41 travel horizontally.

Of course, the depth of the Y-driver latches depends on the number ofbits stored in one memory cell. In the preferred embodiment four bitsare stored in each cell. Therefore, four latches exist per each Y-driver41. For example, in FIG. 4, the Y-driver 0 has four serially connectedlatches 60-63 and the Y-driver 1 has four latches 65-67. Continuingfurther, Y-driver M−1 has the last four latches connected serially. M isthe number of Y-drivers and therefore, the total number of latches is4×M. It is important to note that all the latches are connected acrossall of the Y-drivers 41 of a bank in a long serial link to form a shiftregister. True and complementary outputs of every latch are parallel, asdescribed below with respect to FIG. 6.

The two shift registers, bank A and bank B, are connected throughtransmission switches 145 and 146 to the DATA IN line 25 and DATA OUTline 26, respectively. When a REGSEL control line 147 is high, the DATAIN line 25 and the DATA OUT line 26 are connected to the bank A shiftregister through the switches 145. When the REGSEL line 147 is low, theDATA IN line 25 and the DATA OUT line 26 are connected to the bank Bshift register through the switches 146. The SHFT CLK signal on the line29 clocks the shift registers. With every cycle of the SHFT CLK signal,the data bits move to the next latch. For example, the bit in latch 60moves to latch 61 and the bit previously in latch 61 moves on to latch62 and so on. In the normal operation of the dual shift registers, onebank always operates in the serial mode and the other bank in theparallel mode. The bank which is in the serial mode, receives data from,or reads data out of, the data terminals connected to the DATA IN andDATA OUT lines 25 and 26 serially. At the same time, the other bank inthe parallel mode receives data from, or loads data into, the memorycells of the array 1 in parallel. As the bank in the serial modecompletes its serial operations on the data, the other banksimultaneously completes its parallel operations with the data to andfrom the array 1. Thereafter, the serial bank is switched to theparallel mode and the parallel bank is switched to the serial mode bychanging the state of the REGSEL line 147. This synchronous switchingfrom serial to parallel and vice versa occurs continuously duringwriting into and reading from the memory array 1. Since there are MY-drivers, M memory cells are written in parallel. Since four bits arewritten per cell, a total of 4×M bits are written in parallel. Thisessentially provides a 4×M faster write rate compared to a single bitoperation. similarly, 4×M bits are read in parallel and then shifted outproviding 4×M faster read rates. In fact, the read rate can be performedeven faster by clocking the shift registers at a higher clock rate. Themaximum clock rate is limited by the time required for the parallel datato be loaded into the latches for a serial shifting operations. Hence,as described above, the multilevel dual shift registers block 10 allowsfor faster read and write access times of the memory cell array 1.

The switching between bank A and bank B during both reading and writingoperations can also be non-synchronous. For example, during writingoperations, if the latches of the bank in the serial mode are loadedbefore the latches of the other bank in the parallel mode can programthe memory cells with multiple bits, then the switch of serial andparallel modes between the two shift registers must wait until for thebank in the parallel mode part completes its programming operation.Conversely, if the parallel mode programming operation is completedbefore the serial operations of the first bank are completed, then theparallel mode bank must wait until the serial mode bank is loaded withdata. The same is true for read operations. Thus both synchronous andnon synchronous operations of the dual shift register operation arepossible through the implementation of the appropriate circuitry in thesystem control logic block (shown in FIG. 1). Details of the latches60-63 of the Y-driver0 and latches 64-67 of the Y-driver1 are shown inFIG. 6.

Data Between Dual Shift Registers and Memory Array

FIG. 5 illustrates the organization of the Y-drivers 41 with themultilevel dual shift registers block 10, the read-write block 9 and theY-multiplexer block 8. The individual Y-drivers 41 are each the same interms of operations and circuit detail. Only Y-driver 0 and Y-driver 1are shown. The other Y-drivers up to Y-driver M−1 are represented bydashed lines.

FIG. 7 illustrates the circuit details of a reference multiplexer 50 ofeach read-write block 9 in a Y-driver 41. The true and complementaryoutput signals of each of the latches within a Y-driver 41 are passed toa reference multiplexer 50. Depending on the particular bits in the fourlatches within a Y-driver 41 (in this case, Y-driver 0), the referencemultiplexer 50 connects one of the reference voltage lines, RFL0-RFL15,to the RFLOUT output terminal of the multiplexer 50. Signals on thelines 60A, 61A, 62A, 63A and 60B, 61B, 62B, 63B carry the true andcomplementary output signals, AA, AB, BA, BB, CA, CB, DA and DB,respectively from the four latches of each Y-driver 41, as shown in FIG.6.

The reference multiplexer 50 is essentially a 16-to-1 multiplexer,commonly known in prior art. As apparent in FIG. 7, only one of theRFL0-15 signals appears as the output signal RFLOUT, depending on thesignals, 60A through 63B, from the output terminals 60A-63B of thelatches. Transistors T11 through T164 are N type transistors and theoperation of the multiplexer 50 should be understood. The size of themultiplexer depends oh the number of bits that are being stored in onememory cell. For example, a 6 bit per memory cell storage systemrequires a 64-to-1 multiplexer.

FIG. 8A shows the details of the Voltage Comparator 51, the Latch 52,the Program/Read control circuit 53 and the High Voltage Switch 54 ofthe read-write block 9. The circuitry in FIG. 8A is common to each ofthe Y-drivers 41. The Voltage Comparator 51 has transistors 70-76.Transistors 70 and 71 are P-channel transistors and the rest areN-channel transistors. A VBIAS voltage on a line 198 from the block 23in FIG. 1 provides proper current biasing for the Voltage Comparator 51.The circuit of the Voltage Comparator 51 is known in prior art. Wheneverthe voltage on a signal line 200 to the gate of the transistor 73 ishigher than RFLOUT voltage on the signal line to the gate of thetransistor 72 by even a very small amount, then the SET output on theVoltage Comparator output line 199 is also high, and vice versa. Thegate of the transistor 73 is normally called the non-inverting input andthe gate of the transistor 72 is called the inverting input. The signalline 200 and the signal line 206 described below connect thenon-inverting input to the Y-multiplexer 55. The two lines 200 and 206form a path to read the multiple bits stored in the cells of the array1. The inverting input receives the RFLOUT signal, the output of thereference multiplexer 50, as previously described. The SET output line199 of the Voltage Comparator 51 is connected to an input terminal, thegate of the transistor 80, of the Latch 52.

The Latch 52 has transistors 80 through 85. Transistors 82 and 83 areP-channel transistors and the rest are N-channel transistors. The Latch52 is a classic cross coupled inverter type with an input node, the gateof the transistor 80, connected to the SET output line 199 and anotherinput node, the gate of the transistor 85, connected to the RESET inputline 202. This latch circuit and its operations is well known tointegrated circuit designers. The transistors 81 and 82 form oneinverter and the transistors 83 and 84 form the other inverter. Theoutput node of the Latch 52 is connected by a signal line 201 to theProgram Read Control circuit 53. When the signal on the SET line 199 ishigh or pulsed high, the Latch output on the output line 201 is high.When the RESET line 202 is high or pulsed high, the signal on the Latchoutput line 201 is low. The signals on the SET line 199 and the RESETline 202 are never high at the same time.

The Program/Read Control circuit 53 has two AND gates 88 and 89 and twoinverters 86 and 87. A PROG (program) line 204 is an input to thiscircuit. The signal on the PROG line 204 is high when the write mode isactive, i.e., a writing operation, and is low when the read mode isactive, i.e., a reading operation. When PROG is high (write modeactive), the output of the AND gate 88 depends on the state of theoutput line 201 from the latch 52. If Latch output line 201 is low, thenthe output of the AND gate 88 on the line 205 is high if the PROG signalon the line 204 is high, and vice versa. When the signal on the PROGline 204 is high (write mode active), then the output of the AND gate 89is low. The output line 203 of the AND gate 89 is connected to the gateof a transistor 100. During writing operations, the transistor 100 isturned off and does not allow signals to pass from the line 206, whichis connected to the Y-multiplexer 55, to the line 200. Lines 200 and 206form part of the read path.

The High Voltage Switch 54 has an inverter 90, two N-channel transistors91 and 94, a capacitor 92 and a high voltage transistor 93. The HighVoltage Switch 54 operates as a transmission gate which allows highvoltages on an HV line 209 from the high voltage shaping and controlblock 21 (FIG. 1) to pass to the line 206 when the line 205 is high, orblocks high voltages from the HV line 209 from passing to the line 206when the line 205 is low.

Connected to the read path formed by the signal lines 200 and 206 arethe transistors 101 and 102 which provide the current load to a selectednonvolatile memory cell during reading operations. A VB line 208 is acurrent bias line generated from the Bandgap Reference block 23 (FIG. 1)to the gate of the transistor 102. The transistor 102 operates as asource of the load current during the read mode. The transistor 101 withits control gate connected to a VCTL line 207 acts as a switch to turnthe load current on or off. Inverters 103 and 104 buffer the SET outputon the line 199 from the Voltage Comparator 51 and provides an outputsignal on a READ DATA line 210 during reading operations only. The line210 is connected to its corresponding latches (see FIG. 6) and the line206 to its corresponding Y-multiplexer 55. Thus the transistors 101 and102 act as the bias current circuit 37 and the Voltage Comparator 51acts as the voltage sense amplifier 38 of FIG. 2B for reading operationsin the voltage-mode.

FIG. 8B shows the read-write block 9 of the reference Y-drivers 42. TheVoltage Comparator 51, Latch 52, Program Read Control 53 and HighVoltage Switch 54 are same as that of the Y-drivers 41 for the memoryarray 1, but there are modifications to read eight reference memorycells at a time. During a reading operation, a reference Y-driver 42reads all the reference cells connected to it. Since there are eightreference cells for each reference Y-driver 42 in the presentembodiment, there are eight current loads formed by the transistors 111and 112, each set of transistor output by dashed boxes. The eightVCTL0-VCTL07 lines are forced high to connect the current loads to theirrespective read lines 220-227.

During writing operations, only one of the reference cells is writtento, as selected by the REF Y-multiplexer 56, shown in FIG. 8C, inreference Y-driver 42. Whenever any one of the control lines MCTL0-MCTL7is high, the bit line side RVD FIG. 9A is connected to the read pathlines 260-267 in FIG. 8B.

During a reading operation, all the VCTL0-VCTL07 and MCTL0-MCTL7 controllines are high; this allows all the reference cells to be read inparallel. All VCTL0-VCTL07 control lines high also places the currentloads on the respective read paths of the reference cells. In a readingoperation the READ signal 219 is also high to allow the read voltagefrom the reference cells to be passed to the RFL lines. Eight referencevoltages read back from the reference cells 0-7 are passed to the RFL0-7signal lines respectively through reference Y-driver0 and eightreference voltages read back in parallel from the reference cells 8-15are passed to the RFL8-15 signal lines through reference Y-driver1. Inthe present embodiment it is assumed that the voltages REFB0-15 (FIG.10) are programmed into the reference cells 0-15 respectively. With theREAD signal on the line 219 high, the transistors 211 are OFF and thusthe read back voltage signals do not pass to the Comparator input time200. Notice that transistors 110 and 93 have been placed similarly onall the lines to allow same functionality during a writing operationmode for all the reference cells as occurs to the memory cells in array1 through the Y-drivers 41.

In the reference Y-multiplexer 56 shown in FIG. 8C, each MCTL signaldrives three series transistors M1, M2, M3. This arrangement providessame impedance on the line as provided by the Y-multiplexer 55 for thearray 1, since there are three transistors in series whenever a memoryarray 1 cell is selected by the Y-multiplexer 55. This achieves betterwrite and, more importantly, read mode matching characteristics betweenthe cells of the reference array 2 and the memory array 1. The inverters103 and 104 in FIG. 8A have been removed in the FIG. 8B. This is becausein read operations digital bits are read out from the cells of thememory array 1, whereas reference voltage levels are read out from thecells in the reference array 2.

FIG. 9A shows a Y-multiplexer 55 for the Y-driver 41 for the memoryarray 1. The Y-multiplexer 55 is similar to the reference multiplexer50. In the present embodiment the Y-multiplexer 55 is 8-to-1. The typeof the multiplexer varies (N to 1) depending on the cell size and alsoon the amount of circuitry in the Y-drivers. For the describedY-multiplexer, a single transmission path is connected between the line206 and one of the lines, VD0 through VD7, depending on the Y-addresssignals M0A-M2A and M0B-M2B from the Y-counters. VD0 through VD7 are thecolumn lines in the memory array 1. During program and erase operations,the signals pass from the line 206 to the VD0-7 lines. During a readoperation, signals pass from the VD0-7 lines to the line 206.

FIG. 9A also shows connections to a certain number of the nonvolatilememory cells of the array 1. In this embodiment, one Y-driver driveseight columns and one X-decoder drives four rows of the array 1. Eachrow is considered to be a block in the present embodiment. In otherembodiments, multiple rows may form one single block. The selection ofthe rows by a single X-decoder is performed by the X-multiplexer 58receiving four X-address signals, PA through PD, from the X-counters, asdescribed previously. This basic topology can be extended in both theX-direction to increase the number of rows in the array and in theY-direction to increase the number of columns, in order to increase thesize of the array.

FIG. 9A also shows the reference array 2 and the reference multiplexers56. There are sixteen reference cells from the reference array perblock. Whenever a block is selected through the X-multiplexers 58, bothreference and array cells are selected. The MCTL0-MCTL7 lines drive thereference Y-multiplexer 56. RM0A,B dress sign also drive the inputterminals of the reference Y-multiplexer 56 for the reference array 2,as the M0A,B through M2A,B signals drive the input terminals of theY-multiplexer 55 for the array 1 for each coupled reference Y-driver 42and Y-driver 41.

For the embodiment described here, there are eight times more cellswithin one row than the number being programmed at one time. TheY-drivers 42 and 41 program every eighth cell in a row. A total of eightprogramming cycles are required to program all the cells in a row. Thuscells 0, 8, 16 . . . are programmed in the first programming cycle.Cells 1, 9, 17 . . . are programmed in the second programming cycle andso on. Eight programming cycles program one row. At the same time, thereference cells 0 and 8 are programmed in the first programming cycle.The reference cells 1 and 9 are programmed in the second programmingcycle and so on until eight programming cycles complete the programmingof all sixteen reference cells.

The latches of the REF Y-DRIVER0 and REF Y-DRIVER1 are set to output 0and 8 respectively during the first programming cycle, to 1 and 9respectively during the second programming cycle, and so to set thereference multiplexer 50 of the reference Y-drivers 42 to select theproper RFLOUT voltage at the multiplexer's output terminal from theREFB0-15 voltages provided by the reference generation block 44 shown inFIG. 3. During this writing operation, the latches of the referenceY-driver 42 are internally set to program the appropriate voltages intothe reference cells at the selected locations in the array 2. At thesame time, the latches of the Y-driver 41 are set externally by the datawhich is to be stored in the memory array 1. Of course, the number ofprogramming cycles for a row is dependent upon the ratio of theY-multiplexer. An 8:1 Y-multiplexer requires eight programming cycles,while a 16:1 multiplexer requires 16 programming cycles.

Reading operations from the Memory Array

To further appreciate the voltage mode reading method in the details ofthe circuitry, reference should be made to FIG. 9A. The source linewhich is common for the both array 1 and 2 in the preferred embodimentis connected to a regulated supply voltage Vs. The connection to thetransistor 35 of the cell of the arrays 1 and 2 are shown in FIG. 9B.Assuming that the cell circled and marked XX in the array 1 is beingread. The X-multiplexer 58 selects block 2 through line VG2, also calledthe word line. A word line is connected to the control gate of each ofthe memory cells in the block. The selected word line is connected tothe same supply as is connected to the source, i.e., Vs, or to a voltagethat is high enough to allow the accurate reading of the highestexpected voltage at the column line VD4 with respect to ground. TheY-multiplexer 55 connects the column line VD4 to the line 206. Referringto Fig. 8A now, the line 206 is connected to the line 200 through theturned ON transistor 100. During a read operation, the PROG line 204 islow and reset line 202 is high. This forces the gate 203 of thetransistor 100 to be high to turn the transistor 100 on. The combinationof transistors 101 and 102 form a current source (represented as thebias current circuit 37 in FIG. 2B) between the line 200 and ground. Theline 200 also is connected to the non-inverting input of the VoltageComparator 51 (represented as the voltage sense amplifier 38 in FIG.2B). The transistor 101 acts as a switch for the current source. Thetransistor 101 is only turned on for a short period of time toaccomplish proper voltage comparison by the Voltage Comparator 51. Powerdissipation and also the potential for charges to be trapped in theoxide layer of memory cell transistor is minimized. The RFLOUT input(represented as the reference voltage 39 in FIG. 2B) connected to theinverting input terminal of the Voltage Comparator 51 is the voltageread back from the appropriate reference cell as selected through thereference multiplexer 50 from one of the lines RFL0-15 as shown in FIG.5 and FIG. 7. The result of the comparison at the Voltage Comparator 51is placed on the read data line 210 (represented as the logic output 40in FIG. 2B). During read operations, the high voltage switch 54 isturned OFF and the high voltage line 209 is disconnected from line 206by the high voltage transistor 93.

The dual shift registers, described previously, of the block 10 are usedboth in the write and the read operations in order to reduce the numberof devices in the integrated circuit. The operation of the dual shiftregisters during a writing operation, has been described previously. Ina read operation (refer to FIG. 6), the four latches in a Y-driver 41are preset through the operation of the binary search algorithm. Thesignals BIT3, BIT2, BIT1, BIT0 are forced high sequentially according tothe binary search algorithm shown in FIGS. 12A and 12B. The operationbegins with a RESET pulse on the RESETB line of one bank of themultilevel dual shift registers. The RESET pulse resets all the latcheson one bank of the dual shift registers. According to the binary searchalgorithm, the BIT3 signal is forced high. This sets the line 63A highand the line 63B low for all the latches connected to the BIT3 signalline, latches 0, 4, 8 and so forth), in all the Y-drivers 41. Thevoltage on the RFL8 line of the reference multiplexers 50 is thusselected for the RFLOUT terminal of each Y-driver 41.

At the same time during this read operation, the RFL0-15 lines aredriven in parallel by the voltages read back from the cells of thereference array 2, as described previously. According to the binarysearch algorithm, if voltage read back from the memory cell is higherthan the selected voltage on the RFLOUT within each driver, then thedata output on the READ DATA line 210 in each Y-driver 41 is high. Thisforces the output terminal 601 of the NAND gate 600 low (see FIG. 6),which sets the latch connected to the BIT3 line. The signal at theoutput terminal 602 of the latch remains high even when the data on theline 210 is removed. Once the latch is set, the signals at the outputterminals 63A and 63B remain high and low respectively, even when theline BIT3 is forced low. If the voltage read back from the memory cellis lower than the voltage at the RFLOUT terminal, then the signal on theREAD DATA line 210 is low. This forces the signal at the output terminal601 of NAND gate 600 to stay high and the latch to remain reset. Thuswhen the signal BIT3 is forced low, the signals at the latch outputterminals 63A and 63B would be low and high respectively, the resetstate of the latch. The binary search algorithm continues by forcing theBIT2, BIT1 and BIT0 lines high respectively. A compare operation of thevoltages on the READ DATA line 210 and on the RFLOUT line within eachY-driver 41. The connected latches are set if READ DATA 210 is high orleft reset if RESET DATA line 210 is low. Depending on the set or resetstates of the latches within each Y-driver 41, a different voltage fromthe RFL0-15 lines is selected on the RFLOUT terminal through thereference multiplexer 50 inputs 63A,B to 60A,B (the output of thelatches).

Sequentially four bits from a single memory cell are read into the fourlatches within each Y-driver 41. If N bits were stored per memory cell,then there would be N latches per Y-driver 41 and N bits per y-driverwould be read in N cycles of the binary search algorithm. All the MY-drivers 41 are simultaneously loading their respective latches. Afterthe latches on one bank of the dual shift registers are loaded, the bankis placed in the shift mode and the latched data is then seriallyclocked out from this bank. While the data is being shifted out, theother bank of the dual shift registers is placed in the parallel readmode and the data of another M cells are read into the latches of thisbank. As this bank completes loading its latches, the previous banksimultaneously completes its shifting operation. This alternatingoperation of parallel loading of data from the memory cells and serialshifting of data provides very fast read access times.

During a read operation, the state of the four latches within eachreference Y-driver 42 is not used. The RFLOUT lines are not used withinthe reference Y-drivers 42. Instead, the voltages read from all thereference cells are placed on the RFL0-15 lines as shown in FIG. 8B anddescribed previously.

Writing Operations into the Memory Array

For a write operation, the programming and erase algorithms, as is knownin prior art, typically use a repetitive high voltage pulsed programcycle, followed by a normal read cycle, to set the threshold voltages ofnonvolatile memory cells with a high degree of accuracy. Prior toinitiation of the programming algorithm, an erase pulse of sufficientamplitude and duration is normally applied to completely erase thememory cells. Instead of one erase pulse, some algorithms also repeatthe high voltage erase pulse followed by a read operation as necessaryfor the erase function. In the present invention a single erase pulse isused and then a repetitive high voltage pulse programming algorithm isapplied to accurately set the threshold voltages. Also in the presentembodiment the erase programming and reading occurs on a block basis forfaster write and read access times. Thus M memory cells, representing 4times M digital bits are simultaneously written to or read from.

After an erase cycle has erased all the memory cells in a block, theprogramming cycle is performed. Initially the Latch 52 (detailed inFIGS. 8A, 8B) in each Y-driver is reset by pulsing the RESET 202 line.Thereafter, on a repetitive basis after the application of eachprogramming pulse a read cycle is performed. Within each Y-driver 41 andreference Y-driver 42, the read cycle is performed to determine whetherthe memory cell has reached the desired voltage level set at the RFLOUToutput of the reference multiplexer 50. If the voltage level read backon the line 200 (FIGS. 8A, 8B) has not reached the RFLOUT level, thenthe Latch 52 remains reset and additional high voltage pulses areimpressed upon the memory cell. The source of the high voltage pulses isthe high voltage shaping and control block 21 described in FIG. 1.

During any of the repetitions, if the read back voltage on the line 200is higher than the voltage on the RFLOUT line, the Latch 52 is set andthe high voltage switch of the respective Y-driver 41 (and referenceY-driver 42) is turned off. This stops further transmission of highvoltage pulses to the memory cell connected to that particular Y-driver.It should be understood that while certain Y-drivers may stoptransmission of high voltage pulses to their respectively connectedmemory cells, other Y-drivers may still be passing high voltage pulsesto their respective memory cells in order to program the appropriateread back voltage levels. The read back mode during programming isexactly the same as during the normal read mode, except that the outputsignals on the READ DATA line 210 (FIG. 6) is not stored by the latchesof the block 10. The use of same reading circuitry during programmingand reading modes provides more accurate and reliable data storage andretrieval.

FIG. 10 shows the relationship between the threshold partition referencelevels for the nonvolatile reference array 2 memory cells and for thenonvolatile memory array 1 memory cells. The threshold voltage range tobe partitioned is shown to be from 0V to Vmax. REFA0 to REFA15 are thethreshold partition voltages for the nonvolatile memory array 1 cellsand REFB0 to REFB15 are the threshold partition voltages for thenonvolatile reference array 2 cells. The REFA0-15 levels are midwaybetween the REFB0-15 levels. This ensures accurate and reliablelong-term read comparison of the threshold levels.

FIG. 11 shows the circuit details for threshold partition voltagegeneration blocks for both the nonvolatile memory array 1 and thenonvolatile reference array 2. A bandgap voltage reference unit 300 isan on-chip temperature and power supply voltage source. The operationalamplifier OPAMP 301 is a high gain, unconditionally compensatedamplifier. The circuitry for both the reference unit 300 and theoperational amplifier 301 are known to integrated circuit designers.Resistors 302 through 318 are equal value resistors connected as shown.

The threshold partition generation block for the nonvolatile memoryarray 1 is formed when a resistor 318 is not connected in parallel to aresistor 317, The outputs are called REFA0 to REFA15. When the resistor318 is connected in parallel with the resistor 317, then the nonvolatilereference array 2 threshold partition generation block is formed and theoutputs are called REFB0 to REFB15. Through the programming algorithm,the sixteen reference cells per block in the preferred embodiment areprogrammed to each of the threshold partition voltages REFB0 throughREFB15. The nonvolatile memory array 1 cells are programmed to any oneof the REFA0 through REFA15 threshold partition voltage levels asdefined by the bits in the latches within each Y-driver 41.

For the embodiment described here, there are eight times more cellswithin one row than the number of cells which are programmed at a time.The Y-multiplexers 55 program every eighth cell in a row. A total ofeight program cycles are required to complete programming all the cellsin a row. Thus, cells 0, 8, 16 and so forth are programmed during thefirst program cycle. Cells 1, 9, 17 and so forth are programmed duringthe second program cycle and so on for eight program cycles to completethe programming of one row.

At the same time, the reference cells 0 and 8. are programmed throughthe two reference drivers 42 during the first cycle; cells 1 and 9during the second cycle, and so on, as selected through the referenceY-multiplexers 56. The latches of the REFY-driver0 and REFY-driver1 areset to binary values “0” and “8” respectively during first cycle, andbinary values “1” and “9” respectively during second cycle, and so on toset the reference multiplexer 50 of the reference Y-drivers 42. Themultiplexer 50 selects the proper voltage from the REFB0-15 voltagesprovided by the reference threshold partition voltage generation block44 for the RFLOUT output voltage. In other words, during the writingoperation, the latches of the block 10 of each reference Y-driver 42 areinternally set to program the appropriate voltages into the referencecells at the selected cell locations, while the latches of the block 10of the Y-drivers 41 for the memory array 1 are set externally from thedata to be stored in the memory array 1. The number of program cyclesper row depends on the depth of the Y-multiplexers 55 and 56. Forexample, as described, an 8:1 multiplexer for the Y-multiplexer 55requires 8 program cycles, while a 16:1 multiplexer would require 16program cycles to finish programming a full row.

While various preferred and alternate embodiments of the presentinvention have been disclosed and described in detail, it should beevident that the present invention is equally applicable by makingappropriate modifications to the embodiment described above. Therefore,the above description should not be taken as limiting the scope ofinvention which is defined by the metes and bounds of the appendedclaims.

What is claimed is:
 1. An integrated circuit having an array of memorycells, each memory cell capable of storing a voltage corresponding to aplurality of bits of information, each memory cell having first andsecond terminals, a control terminal and a floating gate, said array ofmemory cells organized into addressable blocks of memory cells, saidintegrated circuit comprising: means for simultaneously erasing aselected block of memory cells; means for coupling a plurality oflatches to said selected block of memory cells; means for selectingreference voltages for each memory cell in the selected block of memorycells based on data in said plurality of latches; and means forsimultaneously programming voltages into each cell of said selectedblock of memory cells responsive to said data in said plurality oflatches, said simultaneously programming means incrementally programmingeach memory cell and comparing a memory cell voltage of said memory cellwith a corresponding selected reference voltage for said memory cellafter each programming increment, said simultaneously programming meansoperative on a memory cell until said memory cell voltage of said memorycell has a predetermined relationship with said corresponding selectedreference voltage for said memory cell.
 2. The integrated circuit ofclaim 1 wherein said simultaneous programming means includes a biascurrent reference generating a bias current through said first andsecond terminals of each memory cell of said selected block, said biascurrent independent of a voltage stored in said each memory cell, andwherein said memory cell voltage at said first terminal correspondsuniquely to said voltage stored in said each memory cell for said biascurrent.
 3. A method of reading a memory cell array in an integratedcircuit, each memory cell capable of holding a voltage corresponding toa plurality of bits, said method comprising: producing a set ofreference voltage levels; and simultaneously reading voltages in aselected plurality of said memory cells with respect to said set ofreference voltage levels to determine a corresponding plurality of bitsin each of said plurality of memory cells.
 4. The method of claim 3wherein said simultaneous reading step comprises independentlydetermining said plurality of bits in each of said plurality of memorycells.
 5. The method of claim 4 wherein said simultaneous reading stepcomprises: generating a bias current through a terminal of each memorycell of said plurality of memory cells, said bias current independent ofa plurality of bits stored in said each memory cell, a voltage at saidterminal corresponding uniquely to a plurality of bits stored in saideach memory cell for said bias current; and comparing said voltage atsaid terminal with said reference voltage levels.
 6. The method of claim4 wherein said simultaneous reading step comprises sequentiallycomparing said corresponding voltage in one of said plurality of memorycells with respect to one of said set of reference voltage levels in anordered sequence to determine each bit in said plurality of bits.
 7. Themethod of claim 6 wherein said reference voltage level producing stepcomprises providing said reference voltage levels from reference memorycells in said memory cell array, said reference memory cells holdingsaid set of reference voltage levels.
 8. The method of claim 7 whereinsaid sequentially comparing step comprises selecting said one of saidset reference voltage levels according to a Binary Search Algorithm. 9.The method of claim 6 wherein said sequential comparing step comprisesdetermining said one of said set of reference voltage levels by apreviously determined bit in said plurality of bits.
 10. The method ofclaim 3 further comprising: storing said corresponding plurality of bitsin each of said plurality of memory cells in a plurality of latches. 11.The method of claim 3 further comprising: organizing into M blockscircuits for reading said memory cell array, each memory cell of saidmemory cell array is capable of holding a voltage corresponding to Nbits; and simultaneously determining N bits in M memory cells in each ofsaid M memory cells.
 12. The method of claim 3 wherein said memory cellscomprise nonvolatile memory cells.